1. Field of the Invention
This invention relates to the field of automatic transmissions of the type used in motor vehicles. The invention pertains more particularly to a transaxle having three planetary gear units whose elements are controlled by overrunning couplings, friction clutches and friction brakes.
2. Description of the Prior Art
Conventional transaxles include a hydrokinetic torque converter and a gearing system disposed coaxially with the torque converter, that axis located parallel to the axis of an axle differential, connected by half shafts to the traction wheels of the vehicle. Torque is transferred from the axis of the gearing to the axis of the differential by a drive chain mechanism driveably engaged with sprocket wheels, one wheel coaxial with the converter axis, the other wheel coaxial with the differential axis.
In transaxles of this type, it is important that various torque transfer elements be arranged so that they occupy minimal space along the axis on which they are located. The axial spacing is minimized to permit the transaxle to be located in a front wheel drive vehicle wherein the engine and transmission are located in a compartment restricted in the transverse or lateral direction by vehicle styling requirements.
When torque multiplication gearing portions of the transaxle are located on an axis parallel to the axis of the torque converter, the packaging considerations are different from those of a transaxle in which the torque transfer elements are located on a common axis with the torque converter and engine Frequently torque transfer elements are located on the axis of the torque output shaft rather than on the engine crankshaft so that the torque delivery gearing is offset laterally or overlaps the engine crank shaft, thereby reducing the overall axial dimension of the transaxle and engine assembly. When the torque delivery gearing is located on a common axis with the torque converter and engine, this overlapping and offset disposition of the gearing with respect to the engine cannot be accomplished. Thus, the problem of reducing the overall axial dimension of the torque converter and gearing, friction clutches, friction brakes and overrunning couplings requires an alternate solution.
In transaxles of this type where a chain drive mechanism is used to transfer torque between parallel axis, the drive chain is located between the torque multiplication gearing and the torque converter. This arrangement unnecessarily increases the overall dimension of the transaction and engine assembly. This shortcoming in the use of available space virtually precludes the use of a third planetary gear set that might be used to produce a fifth forward speed ratio.
U.S. Pat. No. 4,938,097 describes a transaxle having two planetory gear units for producing forward speed ratios and a reverse drive ratio. One of the planetary gear units is nested within the space defined by a sprocket wheel, engaged by a drive chain that transmits torque from the axis of the gearing to the axis of a differential mechanism.